1. Field of the Invention
The present invention relates to a flexural oscillation type actuator used as a speaker for a small terminal device such as a mobile phone, personal computer or the like.
2. Description of Related Art
Conventionally what has been developed is a flexural oscillation type actuator of a bi-morph type or mono-morph type for use in a small terminal device, for example, as a speaker. A known example is a uni-morph type flexural oscillation type actuator in which a beam having a metallic plate referred to as a shim and a piezoelectric layer formed on one surface of the metallic plate is fixed at a central portion thereof to a holding member and configured to expand and contract longitudinally by applying an alternating signal voltage in a direction of thickness of the piezoelectric layer, thereby generating a flexural oscillation in the beam.
Also known is a flexural oscillation type actuator of a bi-morph type in which a piezoelectric layer is formed on both surfaces of a metallic plate (see Japanese Patent Application Laid-Open Disclosure No. 2003-520540, FIG. 9 of a purported International Application).
The Applicant of this application has proposed a bi-morph type flexural oscillation type actuator which comprises a beam including a plate-like member such as a metallic plate and laminated parts including piezoelectric layers and electrode layers which are formed on both surfaces of the plate-like member; one end portion of the beam is held in a case (see Japanese Patent Application No. 2004-263399).
The bimorph-type actuator for flexural oscillation includes two beams 1a and 1b and a case 2 having cavities 3 containing the beams, and has a structure in which one end of each of the beams is fixed to the case 2 by an adhesive, as shown in FIG. 17. Each of the beams 1a and 1b is composed of a shim 5 made of a metallic plate-like member and laminated parts 6 formed on both surfaces of the shim 5. Each of the laminated parts 6 has a plurality of piezoelectric layers and a plurality of electrode layers which are laminated alternately. An alternating current signal is supplied to each beam through electric supply terminals 7 and 8 to extend and contract the piezoelectric layers in a longitudinal direction of the beam so that a flexural signal is induced in each beam.
In this actuator for flexural oscillation, containing the beams 1a and 1b in the case 2 avoids breakage of the fragile piezoelectric layers due to an error by a worker during assembly and also avoids deterioration of the characteristics of each beam through attachment of dust.
Providing the electric supply terminals 7 and 8 on the case 2 prevents the problem that wirings to the beams are complicated and affect oscillation as in the actuator in Japanese Patent Application Laid-Open Disclosure No. 2003-520540 (FIG. 9) of a purported International Application.
Meanwhile, as shown in FIG. 17, inner wall surfaces 9a and 9b of each cavity of the case facing each beam have gradually curved surfaces so that, when an excessive flexural oscillation is caused by, for example, an accidental dropping impact, surface of each beam comes into contact with the inner wall surfaces at a plurality of places in a longitudinal direction of the beam. Excessive flexural oscillation may occur in the beam in a number of cases; for example, when a small mobile device such as a mobile phone installed with the flexural oscillation type actuator as a speaker is accidentally dropped on a floor surface.
In the flexural oscillation type actuator shown in FIGS. 17 and 18, the case 2 made of resin is manufactured as three pieces C1, C2 and C3 and then, in order to hold the beams 1a and 1b between these pieces forming the case 2, the beams 1a and 1b are manually assembled and held in the case by using, for example, a screw mechanism (not shown). Therefore, if the beams 1a, 1b and the case 2 are not arranged parallel to one another or if the beams are misaligned upward or downward, an interval between a surface of each beam and an inner wall surface of the case opposite the beam's surface varies. Consequently, the actuator's flexural oscillation characteristic is deteriorated, since the oscillating beams may come into contact with the inner wall surfaces of the case at irregular intervals.